This invention relates to electrosurgery and in particular to an improved electrosurgical generator.
In electrosurgery there are three effects which may be produced by passing radio frequency current through tissue--namely, desiccation, cutting and fulguration. In desiccation, the active electrode is held in firm contact with the tissue with the current passing directly into the tissue and the heating effect being brought about by I.sup.2 R heating. Thus, the mode of operation is ohmic. A generator designed for desiccation should be able to pass at least one ampere through 100 ohms.
In cutting, the active electrode is not in good contact with the tissue and electric sparks jump from the electrode to the tissue. The voltage waveform used is generally a sinewave and the sparks are short in length but heat the tissue intensely. The cells burst into steam and the steam maintains the layer of gas between the electrode and the tissue as the incision proceeds. In order to produce a cutting effect without desiccation, the generator must be current limited to less than about 200 milliamperes. Since the open circuit voltage must be high to make a spark, the output impedance must be high, about 3000 ohms. About 100 watts power is sufficient.
In fulguration, the coagulation occurs by means of a high voltage spark which jumps from the active electrode to the tissue. Thus, an arcing mode of operation occurs in both cutting and fulguration. The spark produces intense heating at every point it strikes, but a high crest factor voltage waveform, called a COAG waveform, makes long sparks and distributes the spark widely. This keeps the energy density down and minimizes the cutting effect. A generator designed for fulguration without cutting or desiccation effects needs a very high output impedance, say 5000 ohms or more and should be current limited to less than 200 milliamperes. About 125 watts is adequate.
At present, some general purpose generators have high power at the low impedances, around 100 ohms and similar amounts at high impedances, 3000 to 5000 ohms. In order to achieve this, some generators operate most efficiently into a 400 or 500 ohm resistance, see FIG. 1, General purpose curve. In order to provide the necessary power at low and high impedances, the generator is excessively powerful at 500 ohms. 400 or more watts are required. Desiccation and fulguration or cutting cannot occur simultaneously, so no more than 125 watts is needed at any one instant.
It is thus a primary object of this invention to provide an improved multiple source, electrosurgical generator having at least a separate desiccation source and a separate source for fulguration and cutting where separate sources may also be respectively provided for cutting and fulguration.
It is to be understood the foregoing generator of this invention is different from those prior art generators such as those disclosed in U.S. Pat. Nos. 1,945,867 and 3,058,470 where a spark gap generator (for fulguration) is connected to the same output as a cutting waveform generator. Thus, both of the foregoing modes of operation (fulguration and cutting) are arcing modes in contradistinction to the generator of the present invention where one of the sources operates in and may be optimized for an ohmic mode of operation (desiccation) while the other source operates in and may be optimized for an arcing mode of operation (cutting or fulguration).
By controlling the voltage across the load (tissue) and the current through it, it is possible to confine the electrosurgical effects (arcing or ohmic mode) to whatever is desired. There are known ways to do this and the following attempts to summarize what has been done. At present there are at least four basic, commercially available, output impedance characteristics. These four characteristics accentuate the transfer of power to different regions of the impedance spectrum to enhance different performance. A typical general purpose generator, as discussed hereinabove, may have an output impedance of 500 ohms. This impedance is mainly a function of the number of turns on the secondary winding of the output transformer. The characteristic is rather sharp, that is, it peaks at 500 ohms and falls off rapidly at impedances either above or below that figure. By using low output power settings, and pressing the active electrode against the tissue, it is possible to stay in the desiccation mode and avoid cutting or fulguration. By using higher settings and holding the electrode just off the tissue, one can achieve cutting or fulguration depending on waveform.
In most electrosurgery, desiccation is combined with cutting or fulguration because the surgeon usually starts his cut or fulguration with the electrode in firm contact with the tissue. Since, by definition, the starting mode is desiccation, the desiccation must be complete before the tissue in contact with the electrode will acquire a high enough resistance so that sparking can begin and cutting or fulguration will occur. Thus, for example, in many cutting applications, the cutting electrode must be pressed against the tissue before activating the generator. As a result, the electrode tends to desiccate instead of cut and the surgeon sees this as a delay or a lag. To overcome this, some generators use a "rapid start" characteristic which means fewer turns on the secondary so that the peak power point is at about 250 ohms, see Rapid Start curve of FIG. 1. This characteristic provides additional power to the desiccation part of the spectrum so that the electrode is rapidly driven into high impedances and will begin to spark and cut. The drawback of this characteristic for general use is that the open circuit voltage is reduced and fulguration from a distance is degraded.
For neurosurgery and other applications, it is desirable to have an output that will desiccate only, without any possibility of fulguration or cutting. A low impedance output winding with a peak power at about 100 ohms accomplishes this, see the Microbipolar curve of FIG. 2 and the Desiccation Only curve of FIG. 1.
A recently available characteristic is a more flat general purpose characteristic which, instead of peaking at 500 ohms, is flat from about 100 to 1000 ohms, see the Monopolar curve of FIG. 2. This characteristic combines most of the advantages of the standard 500 ohm peak characteristic with the "rapid start". It may be effected by a feedback loop and a voltage clamp which regulate the voltage and current in the output transistors.
At present, a generator which is optimized for all three functions, desiccation, cutting and fulguration, must use at least two secondary transformer windings and, without switching very high voltages, it is not practical to have all three capabilities available with the same hand-held electrode simultaneously.
It is thus a further primary object of the present invention to provide an electrosurgical generator of the foregoing type wherever the different modes (ohmic or arcing) of operation may be optimized and may be effected in a common output circuit whereby all modes are readily available from a hand-held switching, electrosurgical instrument having a plurality of switching positions to permit the surgeon to readily select the desired mode or combination of modes of operation.
As stated above there are certain applications where a desiccation only mode of operation is desirable such as in neurosurgery. Further, there are applications where a fulguration only or cutting only is desired. Thus, for example, one practical application of fulguration-only is in the fulguration of bleeding gastrointestinal ulcers. This procedure is done through an endoscope and because the wall of the stomach or bowel is very thin, it is desirable that the coagulation necrosis be confined to the surface of the tissue. Because desiccation causes deep coagulation necrosis and the depth and effectiveness are very dependent on the pressure one exerts on the electrode, controlling this pressure through an endoscope is very difficult. Fulguration provides very consistent, shallow necrosis with rigid crust that can stop even very large arterial bleeders. This works well, but if the electrode inadvertently touches the wall of the bowel, desiccation may occur and deep necrosis may result. By limiting the coagulation to fulguration, this possibility is avoided and the technique requires less skill to perform.
It is thus a further primary object of this invention to provide improved electrosurgical generators which operate in a desiccate only mode of operation or a fulgurate only or cut only mode of operation.
It is a further object to provide a multiple source generator of the type described hereinbefore where one of the sources operates in a desiccate only mode and at least one of the sources operates in an arc (fulguration or cutting) only mode.
As stated above, a fulgurate only (no desiccation) characteristic is desirable in some applications. However, as also indicated hereinbefore, some desiccation is needed to initiate the establishment of the arc required for fulguration. Further, as in some instances a "rapid start" cutting action is desired.
Hence, it is a further object of this invention to provide an improved multiple source generator of the aforesaid type where a desiccate only source and a cut only and/or fulgurate only source(s) are independently adjustable whereby the desiccate power level can be adjusted with respect to the cut only or fulgurate only power level to provide sufficient desiccation to initiate the requisite arc either with a slight delay or on a "rapid" start basis. Prior art electrosurgical generators are known wherein control of the generator output is effected in accordance with sensed tissue impedance. Thus, in British Pat. No. 855,459 there is disclosed a voltage feedback circuit which turns the generator off when a predetermined level of "coagulation" of the tissues has occurred, the coagulation possibly being due to desiccation as opposed to fulguration. Hence, there may be disclosed in the British Patent a desiccation only circuit using voltage feedback. It is also known as described in U.S. Pat. Nos. 3,923,063 and 4,024,467 to vary the duty cycle of the applied signal by a voltage feedback circuit to prevent cutting during coagulation, which does not correspond to the prevention of desiccation. Also in U.S. Pat. No. 3,601,126 there is disclosed a current feedback circuit for regulating the output power of an electrosurgical generator. However, in none of the foregoing patents is there disclosed an arc (cut or fulgurate) only mode of operation or the combination of desiccate only and arc (cut or fulgurate) only modes of operation discussed hereinabove. Further, as will be described hereinafter, it has been empirically determined in accordance with the present invention that positive current feedback is effective for implementing a desiccate only mode of operation.
Thus, further objects of this invention are (1) to provide an arc only mode of operation and (2) to employ positive current feedback to implement a desiccate only mode.
Another factor relating to multiple source generators is discussed in my copending application entitled "Patient Contact Area Measurement Apparatus and Method For Use In Electrosurgery and Cryosurgery" filed Nov. 17, 1977. In this application is described a capacitive, split patient electrode which, together with the patient electrode cable may comprise a resonant circuit to minimize the voltage between patient and ground where the split patient electrode comprises a pair of separated plates both of which are connected to an electrosurgical generator via the patient electrode cable, there being disposed on the plates a dielectric layer adapted for contact with the patient. In the capacitive split patient electrode patent application, it is stated the dielectric has to typically withstand 5000 volts and the electrode has to be a relatively small size so the capacitance could not be very large, typically 3000 to 4000 pF. As a result, the operating frequency has to be high to pass large desiccation currents. Otherwise, large voltages would appear between the patient and ground. Even with the patient electrode and cable resonating at the operating frequency, it has been found the application of the patient electrode becomes quite critical when an attempt is made to resonate the cable at low frequencies, say 750 KHz. At 2.2 MHz, for example, however, the application is not critical and patient contact area typically in excess of 60% provides a sufficiently low patient to ground impedance.
From the foregoing it can be seen the higher operating frequency of 2.2 MHz is desirable when using an insulated (dielectric) electrode to permit effective handling of the larger current associated with the desiccation (ohmic) mode of operation. Smaller currents are associated with cutting and fulguration (arcing) modes of operation and thus the problem of a high potential between the patient and ground is not as significant. Hence, cutting and fulguration can occur at a lower frequency of operation.
Thus, it is a further object of this invention to provide an electrosurgical generator having separate sources of the above type where a higher frequency of operation is employed for desiccation to thereby permit the use of an insulated, capacitive electrode which may be of the split type.
As discussed hereinbefore, a fulguration only mode of operation is desirable in certain applications whereby the patient current is limited to a predetermined value such as 200 ma. to thereby prevent desiccation. However, in some applications, it is desirable to employ needle electrodes in the fulguration only mode of operation. The current density through such a needle electrode is quite high due to the small contact area thereof with the treated tissue. Hence, the current should be limited to a value even less than 200 milliamperes in these applications.
In accordance with a further object of this invention, improved electrosurgical circuitry is provided for ensuring a fulguration only mode of operation even when a needle electrode is employed. In order to effect the foregoing, spark detection circuitry is employed in a preferred embodiment of this invention to detect the presence of sparking.
The foregoing spark detection circuitry is useful in other applications. The operation of the spark detection circuitry of this invention relies upon the stochastic nature of the sparking phenomenon. In particular, the very low frequency portion of the spectrum is detected in accordance with a further object of this invention, the low frequencies corresponding to those capable of producing neuromuscular stimulation or even shock or fibrillation. Spark detection circuitry and techniques are disclosed in U.S. Pat. Nos. 3,163,165 and 3,515,943, German Pat. No. 2,504,280 and French Pat. No. 1,275,415. However, none of the foregoing patents include circuitry sensitive only to the low frequency portion of the sparking spectrum.
In accordance with a further object of this invention, the aforesaid spark detection circuitry may also be employed to detect lack of circuit continuity in the patient circuit during fulguration (or cutting) mode of operation where the current is very low. This is in contradistinction to the process disclosed in U.S. Pat. No. 3,812,858 which simply discloses circuitry for sensing lack of power in the patient lead, the circuitry being insensitive to whether fulguration (or cutting) is occurring.
Other objects and advantages of this invention will be apparent from a reading of the following specification and claims taken with the drawing.